1. Field of the Invention
The present invention relates to a rotary electrical machine and an electric vehicle having the same.
2. Description of the Related Art
Conventional, rotary electrical machines have a rotor, a rotational shaft attached to the rotor, and a stator. The strength of the magnetic field between the rotor and the stator is controlled in response to a rotational speed of the rotor. In this kind of rotary electrical machine, the characteristic of the rotary electrical machine (such as, for example the relationship between the rotational speed and the output torque of the rotational shaft) can be adjusted by controlling the strength of the magnetic field.
One such rotary electrical machine is disclosed, for example, in Japanese Publication No. JP 11-122886. This rotary electrical machine includes a housing, which is a stationary side member and forms an outer shell of the rotary electrical machine, and a rotational shaft journaled on the housing for rotation about an axis thereof, but not for movement in the axial direction thereof. The rotary electrical machine also includes a rotor coupled with the rotational shaft to rotate therewith about the axis, a stator supported by the housing so as to face the rotor, and a centrifugal governor joined to a distal end of the rotational shaft. A yoke, which forms the stator, has multiple receiving apertures that extend parallel to the axial direction of the rotational shaft (and can be simply described as “extending in the axial direction” below). Auxiliary yokes, each of which extends in the axial direction of the rotational shaft are inserted into corresponding receiving apertures. Each auxiliary yoke is connected to the centrifugal governor. The centrifugal governor moves the associated auxiliary yoke in the axial direction. That is, an inserted length of each auxiliary yoke is variable in accordance with the rotational speed of the rotational shaft. Thus, the movement of the yoke varies in response to the rotational speed of the rotational shaft, and the strength of the magnetic field established between the rotor and the stator can be adjusted.
When the rotary electrical machine operates as an electric motor, an electric current is supplied to the stator so that the rotor rotates. With the rotation of the rotor, the rotational shaft generates output torque. In the rotary electrical machine described above, if the rotational shaft rotates at high speed, the magnetic field is weakened because of an action of the centrifugal governor. As a result, the output torque per unit current (i.e., output torque of the rotational shaft generated per unit current flowing through the stator) decreases. As thus discussed, by weakening the magnetic field, the rotational shaft can rotate at a high speed under a low torque condition.
Conversely, when the rotational shaft rotates at a low speed, the magnetic field can be strengthened by the centrifugal governor operating in association with the rotational shaft. As a result, the output torque per unit current increases. Thus, by strengthening the magnetic field, the rotational shaft can rotate at a low speed under a high torque condition. That is, in the rotary electrical machine described above, when it operates as an electric motor, a torque characteristic can be obtained in which the rotational speed increases while the output torque of the rotational shaft is small, and also the output torque increases while the rotational speed is low.
Japanese Publication No. JP 03-215154 discloses a rotary electrical machine in which the strength of the magnetic field between a rotor and a stator is adjusted by urging the rotor toward or away from the stator. This rotary electrical machine includes a stationary side member and a rotational shaft journaled on the stationary side member for rotation about an axis thereof. The rotor is coupled to the rotational shaft to rotate about the axis and the stator is supported by the stationary side member so as to face the rotor. A spring urges the rotational shaft in a direction in which the rotor recedes from the stator, and an electromagnet is electrically connected to the stator in series. The rotational shaft is formed of a magnetic material. In this rotary electrical machine, when an electric current is supplied to the electromagnet, the electromagnet attracts the rotational shaft in the axial direction. If the attractive force of the electromagnet is larger than the urging force of the spring, the rotor approaches the stator. Conversely, if the attractive force of the electromagnet is smaller than the urging force of the spring, the rotor recedes from the stator. As thus discussed, the electromagnet and the spring together form an actuator that moves the rotational shaft in the axial direction.
When the rotary electrical machine described above operates as an electric motor, an electric current is supplied to the stator and the electromagnet to rotate the rotor. If the load applied to the rotational shaft is small, the current supplied to the stator is small. Thus, the current supplied to the electromagnet is also small, and the electromagnetic force of the electromagnet is small. Accordingly, the force of the electromagnet that attracts the rotational shaft is smaller than the urging force of the spring, and the rotor is kept separate from the stator. Consequently, the gap size between the rotor and the stator is large to maintain the magnetic field therebetween weak.
On the other hand, as the load applied to the rotational shaft increases, the current supplied to the stator increases. Thus, the current supplied to the electromagnet also increases, and the attractive force of the electromagnet becomes larger than the urging force of the spring. Because of the attractive force of the electromagnet, the rotational shaft moves in the direction in which the rotor approaches the stator. Consequently, the gap size between the rotor and the stator decreases to strengthen the magnetic field therebetween.
Thus, according to the rotary electrical machine disclosed in Japanese Publication No. JP 03-215154, the same torque characteristic as that of the rotary electrical machine disclosed in Japanese Publication No. JP 11-122886 can be obtained. That is, when the rotary electrical machine disclosed in JP 03-215154 operates as an electric motor, a torque characteristic can be obtained in which the rotational speed is high while the output torque of the rotational shaft is small, as well as one in which the output torque is large while the rotational speed is low.
The rotary electrical machine disclosed in Japanese Publication No. JP 11-122886 needs a centrifugal governor attached to the rotational shaft to obtain the torque characteristic discussed above. However, because the centrifugal governor generates the centrifugal force using a weight, the governor needs some structure that moves the weight in a radial direction of the rotor. In order to ensure a space for the movement of the weight, the rotary electrical machine needs to have a certain capacity in the radial direction. Accordingly, a problem arises in that the external size of the rotary electrical machine is inevitably large. Also, because the receiving apertures, the auxiliary yokes and the like described above are necessary in addition to the centrifugal governor, another problem arises in that the structure of the rotary electrical machine is complicated.
The rotary electrical machine disclosed in Japanese Publication No. JP 03-215154 needs to have the above-described actuator provided outside of the rotational shaft in the axial direction. The length of the rotary electrical machine in the axial direction can be longer correspondingly to the length of the actuator. Thus, the size of the rotary electrical machine is inevitably large. A large space thus is needed for mounting the rotary electrical machine.